The present invention relates to an audio system, and more particularly to a system which enables to provide a uniform sound field characteristics throughout the entire listening space.
There has been known an audio system wherein:la plurality of loudspeakers are disposed in a room, thereby to provide a uniform sound field characteristics in the room.
Referring to FIG. 4, a plurality of loudspeakers 5 of an example of the conventional audio system for a motor vehicle are provided in a dashboard at the both sides thereof, doors 2 and 3, or in a rear package shelf 4. By driving; these loudspeakers 5, a uniform sound field is to be presented to passengers 6.
In order to create a uniform sound field with the conventional audio system described above, providing a plurality of speakers is not enough. It is preferable to dispose the speakers in an interior space where the wave characteristic of the sound waves such as reflection and composition can be ignored, that is, idealistically speaking, the free sound field.
However, in a limited space of the motor vehicle, the wave characteristic of the sound waves cannot be ignored. The direct sound emanated from each speaker and the reflected sound reflected from the wall of the interior is composed with each other, thereby generating a reflection composite effect. As a result, the composite sound causes the frequency response of the original music emanated from the speakers to be impaired, thereby generating a frequency component offensive to the ears called a peak or a dip in the audible frequency range.
In order to resolve such a problem, there has been proposed an audio system using a minimum number of speakers, thereby reducing the number of the sound sources themselves, which is the cause of the reflection composite effect. For the case, the speakers are disposed only at the sides of the dashboard. However, although the number of the speakers is decreased, in a space having a complicated shape such as the interior of a motor vehicle, the reflection composite effect cannot be sufficiently restrained. Due to the complicated shape of the interior space of the motor vehicle, a peak of a low frequency component is generated as a sound field characteristic, and in addition, the sound pressure level of the low frequency component is higher at a position adjacent the rear seat than at a position adjacent the front seat. As a result, the passengers at the rear seat hear a disagreeable muffled sound.
An object of the present invention is to provide an audio system which solves the problems of the conventional audio system, thus providing a uniform sound field wherein the influence of the reflection composite effect is restrained in the entire space.
According to the present invention, there is provided an audio system for a room comprising, a sound source, a first speaker for emitting a first sound including a reflected composite signal based on an audio signal from the sound source, a second speaker for emitting a second sound, a microphone disposed at a listening position between the first and second speakers for picking up the first sound from the first speaker and the second sound from the second speaker, control means for obtaining a time difference and a sound pressure difference between the reflected composite signal and the second sound picked up by the microphone based on characteristic information of the first and second sounds, phase adjusting means for inverting the phase of the audio signal from the sound source for producing an antiphase audio signal, filter means for extracting an input audio signal of a predetermined frequency band from the antiphase audio signal, in which the predetermined frequency band corresponds to a frequency band of the reflected composite signal included in the first sound picked up by the microphone, delay means for delaying the antiphase audio signal bas ed on the time difference so that the second sound coincides with the first sound at the microphone, amplitude adjusting means for adjusting the amplitude of the antiphase audio signal so as to reduce the second pressure difference, applying means for applying the inverted antiphase audio signal to the second speaker for emitting the second sound.
The present invention further provides an audio system for a room comprising, a sound source, a first speaker for emitting a first sound based on an audio signal from the sound source, a second speaker for emitting a second sound, phase adjusting means for inverting the phase of the audio signal from the sound source for producing an antiphase audio signal, filter means for extracting an input audio signal of a predetermined frequency band from the antiphase audio signal, in which the predetermined frequency band corresponds to a frequency band of a reflected composite signal included in the first sound applied to a listening position, delay means for delaying the antiphase audio signal a delay time obtained based on a time difference between a first time lag between the audio signal from the sound source and the first sound applied to the listening position, and second time lag between the audio signal from the sound source and the second sound applied to the listening position, amplitude adjusting means for adjusting the amplitude of the antiphase audio signal so as to approximate to the second pressure of the first sound, applying means for applying the inverted antiphase audio signal to the second speaker for emitting the second sound.